The National Transportation Safety Board has also interviewed a pilot who was refueling his helicopter at west 30th street. That pilot saw the Piper Lance heading toward the helicopter, and tried to warn the chopper pilot.

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"He was obviously concerned enough to radio to let him know about the close proximity of the other traffic," said NTSB chairwoman Deborah Hersman.

Onboard the helicopter were five Italian tourists and a pilot, embarking on what was expected to be a brief 12-minute tour from the air. The aircraft, owned by Liberty Helicopters, had barely taken off from Manhattan when it collided with a Piper Lance plane, sending both plunging into the Hudson.

Still, questions remain about the cause and what could have been done to prevent the deadly accident.

Investigators believe that after the Piper left Teterboro airport, the pilot -- Steve Altman -- may have failed to follow instructions to switch to the Newark airport frequency and check in with controllers there. Hersman said the Teterboro airport then tried to contact the aircraft but it was unsuccessful.

A government source also told ABC News that the plane -- flying at around 1,100 feet -- was apparently making a climbing right turn when it crashed into the helicopter. In that area over the river, aircraft flying below 1,100 feet are outside the supervision of air traffic controllers, and simply required to avoid each other.

In other words, below 1,100 feet, pilots are virtually on their own with no controllers guiding them in the crowded airspace. One pilot called this area the major leagues of aviation, and said pilots should not play here if they are not proficient. Hersman said it's the responsibility of pilots "to see and be seen and be aware of traffic around them."

Calls For More Aviation Restrictions in Low-Altitude Hudson Corridor

Pilot Paul Dudley has flown over the Hudson for 25 years, and he said pilots are required to watch out for other aircraft and avoid collision by communicating on a common frequency.

"So if you were to listen in on that radio you'll hear all the different helicopters and aircraft calling in reporting who they are, how high they are and where they are," Dudley told ABC News. "See and avoid. This is not magic."

But the practice has some people today turning up the political pressure. Today Rep. Jerrold Nadler, D-N.Y., who serves on the House Transportation and Infrastructure Committee, demanded the Federal Aviation Administration immediately regulate the city's congested and dangerous airspace.

"It is unconscionable that the FAA permits unregulated flights in a crowded airspace in a major metropolitan area," Nadler said. "And it is ridiculous that private planes and helicopters flying through a crowded area are dependent, while in flight, on visually sighting other aircraft and communicating with them. The real-life repercussions of these non-existent regulations have been disastrous."

City Council Speaker Christine C. Quinn said her office would hold hearings on the issue.

"Regardless of what the investigation uncovers, the time has come for the FAA to reassess their regulatory practices for the Hudson River corridor," Quinn said. "And it's time for the City to review and analyze our policies when it comes to air traffic over our neighborhoods."

"I'm not going to pressure the FAA," Mayor Michael Bloomberg said Monday. "They don't need me weighing in. They know certainly well what goes on there. They are professionals. I assume they're going to wait until the National Transportation Safety Board to make its report and then they'll make their decisions."

She also said midair collisions -- 11 per year in the last decade -- have resulted in 158 deaths.

"One midair a year is too many, and the Board has seen 11 midairs a year for the last decade," Hersman said. "And so we are really very concerned about what kind of procedures, technologies, training, might be available to prevent these kinds of accidents."

"Certainly there is a common theme of people not being aware of each other," she also said.

Meantime, victims' families continue to mourn their loved ones. Accompanying Altman on the Piper were his brother and nephew. The helicopter was being flown by a 32-year-old pilot -- Jeremy Clark of Lanoka Harbor, N.J. -- and five Italian tourists, including a couple and their teenager, and a father and son, whose mother stayed behind because she was too nervous to climb on board.